C-13 ENRICHMENT IN BENTHIC COMPARED TO PLANKTONIC ALGAE - FOODWEB IMPLICATIONS

Previous work has shown freshwater macrophytes from lake Littoral zones to be C-13-enriched compared to the same species collected from fast moving rivers. It is thought that carbon fixation in aquatic plants having thicker, stagnant boundary layers, such as that which occurs within low turbulence lentic systems, will result in more positive delta(13)C values due to greater diffusion resistance and subsequent assimilation of otherwise normally discriminated C-13. The present study confirms this hypothesis by examining 876 algal delta(13)C values collected from the Literature. The average delta(13)C value far benthic algae in lakes was similar to 26 parts per thousand, whereas that for riverine benthic algae was -29 parts per thousand. The greater water turbulence to which planktonic algae are exposed is known to dramatically reduce boundary layer thickness and was found to cause even more severe C-13 depletion, resulting in an average value of -32 parts per thousand. This same effect also operates in coastal environments where the average delta(13)C value for marine phytoplankton was -22 parts per thousand compared to -17 parts per thousand for marine benthic algae. When comparisons were made on an individual study basis, differences of 10 parts per thousand or greater in delta(13)C were observed between planktonic and benthic algae in both oceans and lakes. These algal differences in delta(13)C were found to be substantial enough to be reflected in the delta(13)C values of consumers in marine coastal environments.